/// <summary>
/// Adds a key/value pair to the <see cref="IDictionary{TKey, TValue}"/>
/// if the key does not already exist, or updates a key/value pair in the <see cref="IDictionary{TKey, TValue}"/>
/// by using the specified function if the key already exists.
/// </summary>
/// <typeparam name="TKey"></typeparam>
/// <typeparam name="TValue"></typeparam>
/// <param name="dictionary">The dictionary which the key should be added to or updated in.</param>
/// <param name="key">The key to be added or whose value should be updated</param>
/// <param name="addValue">The value to be added for an absent key</param>
/// <param name="updateValueFactory">The function used to generate a new value for an existing key based on the key's existing value</param>
/// <returns>The new value for the key. This will be either be addValue (if the key was absent) or the result of updateValueFactory (if the key was present).</returns>
/// <exception cref="System.ArgumentNullException">key or updateValueFactory is null.</exception>
/// <exception cref="System.OverflowException">The dictionary already contains the maximum number of elements (<see cref="System.Int32.MaxValue"/>).</exception>
public static TValue AddOrUpdate <TKey, TValue>(this IDictionary <TKey, TValue> dictionary, TKey key, TValue addValue, Func <TKey, TValue, TValue> updateValueFactory)
{
dictionary.ThrowIfNull(nameof(dictionary));
key.ThrowIfNull(nameof(key));
updateValueFactory.ThrowIfNull(nameof(updateValueFactory));
TValue oldValue;
TValue newValue;
if (dictionary.TryGetValue(key, out oldValue))
{
newValue = updateValueFactory.Invoke(key, oldValue);
dictionary[key] = newValue;
}
else
{
newValue = addValue;
dictionary.Add(key, newValue);
}
return(newValue);
}